The existence of regular boundary points for non-linear elliptic systems

We consider non-linear elliptic systems of the type
\[
-\mbox{div}\, \ a(x,u,Du)=0
\]
with H\"older continuous dependence on $(x,u)$, and give
conditions guaranteeing that $H^{n-1}$-almost every boundary
point is a regular point for the gradient of solutions to related
Dirichlet problems. We also introduce a new comparison technique,
in order to deal with difference quotients.